Solvent-extraction processes used for the recovery and refining of metals are well known and reference is made to the disclosure of Gallacher, U.S. application Ser. No. 671,346, filed Mar. 29, 1976; Swanson, U.S. Pat. Nos. 3,224,873; Swanson, 3,428,449; Hazen and Coltrinari, 3,872,209; and Morin and Peterson, 3,878,286, each of which is incorporated herein by reference. Such processes typically involve continuous recycling of three streams: an aqueous leaching solution which contains the desired metal value, a water-insoluble organic extractant, and an aqueous stripping solution which recovers and concentrates the metal value for electrical or chemical removal later.
In a typical process for refining copper, a low-grade copper ore is leached by dilute sulfuric acid to produce a leach solution containing 2-4 grams per liter of copper and approximately the same concentration of ferric (Fe.sup.+3) iron. The aqueous leach solution is then contacted with an organic extractant in kerosene at a 1:1 phase volume ratio in a counter-current sequence of mixer-settlers to produce an organic extract containing almost all of the copper from the original aqueous leach and a negligible amount of the iron. The organic extract in turn is contacted with a sulfuric acid acid stripping solution or "spent electrolyte" containing about 30 grams per liter copper at a very low phase volume ratio to generate an aqueous "loaded electrolyte" containing about 50 grams of copper per liter. The loaded electrolyte is passed through an electrowinning cell to produce cathode copper and simultaneously regenerate the stripping solution.
In the operation of solvent-extraction metal refining processes, it is apparent that extraction of species other than the desired metal value must be minimized, as well as other losses, e.g., formation of insoluble complexes. It is also apparent that phase separation rates in the extraction and stripping steps should be rapid in order to minimize the volumes of separation vessels and thereby reduce the solvent inventory and equipment costs necessary to construct and start up a solvent-extraction process plant.
Thus, it is important that after extraction and stripping, the separated organic and aqueous phases contain minimum levels of suspended particles or droplets of the second phase. Each phase should be as clear as possible with minimum haze. This haze is commonly referred to as "secondary" haze because it persists after primary phase disengagement is complete. In general, the organic extractant is considerably more valuable than either the leaching or stripping solutions, and, therefore, haze in the aqueous phases after extraction or stripping is particularly undesirable, since it represents potential extractant losses. A secondary haze of only 0.001% in the separated aqueous phase after extraction in a process which recycles the organic extractant every 30 minutes will result in an extractant make-up requirement of approximately 15% per year.
It is obvious that extractant losses due to aqueous solubility can be just as critical as losses through second-phase dispersions.
The formation of insoluble species, particularly in the organic phase, is also to be avoided. Such species can form in several ways, including simple protonation of the active extractant to form an insoluble salt, e.g., a bisulfate, or by formation of an insoluble metal complex.
Dinonylnaphthalene sulfonic acid (DNNSA) is known to be an effective cation extractant, particularly under strongly acid conditions where other acid extractants fail to function. As U.S. Pat. No. 4,018,865 shows, DNNSA can be combined with aliphatic alpha hydroxy oximes to produce highly selective extractants. In the same application it is shown that the closely related didodecylnaphthalene sulfonic acid (DDNSA) can also be combined with aliphatic alpha hydroxy oximes to produce highly selective extractants.
Surprisingly, it has now been found and is the subject of this invention that extractant solutions containing either DDNSA alone or in combination with aliphatic alpha hydroxy oximes are consistently superior to the corresponding DNNSA extractants in one or more of the critical characteristics described above.